So what is the link between circle of fifths and Songwriting cards ?
Let's choose the C (major) chord. According to the circle of fifths theory, we can play G (major), F (major), Dm (minor), Em (minor) and Am (minor) chords.
Now, lookk at the C (major) chord card from Songwriting cards, the right sidebar list the same chords with the related degrees.
That's it, for every given chord, you see at first glance, all related chords based on the Circle of Fifths theory.
Read more: how to read cards